The present invention generally pertains to echo cancellation in a two-way communications network and is particularly directed to echo cancellation in a digitized speech signal communication system containing a digital voice codec, such as a RELP vocoder.
Echoes in a communications network are caused when an impedance mismatch in a 4-wire circuit allows the coupling of transmit and receive data paths. In two-way communication networks, it is not uncommon for a received signal to be echoed onto a transmit channel. For example, practical telephone comminications networks use two-wire telephone handsets, which are coupled to four-wire telephone transmission lines by conversion hardware that sometimes is not properly impedance matched; and in such instances an echo return loss (ERL) in the range of 6 to 12 dB is typical. As a result, with a voice input/output communication system, such as a telephone network, in which an echoed signal is produced, a person may hear an echo of his own speech if the round-trip delay incident to the return of the echoed signal is significant, such as greater than 40 msec. In conventional telephone networks, echo cancellation is required only for long-haul trunks where significant transmission delays are present.
However, with the advent of communications networks in which digitized voice signals are produced and transmitted through utilization of digital voice codecs and speech compressors, the round-trip transmission delays become more significant over even relatively short geographical distances when the transmission is at a relatively low bit rate, such as less than 16 Kbps, whereby echo cancellation is required.
A classical echo canceller for a communications network, such as a telephone network, is described in a publication by M. Sondi and D. Berkley, "Silencing Echoes on the Telephone Network", Proc. IEEE, 1980, pp. 948-63. This echo canceller sythesizes an estimate of the echoed signal and subtracts the estimate from the combined signal on the transmit channel. The estimate is generated by filtering the received signal that is being echoed with adaptation coefficients. The adaptation coefficients may be generated by an algorithm in which the coefficients are iteratively updated from a correlation of the received signal with the difference of the estimate from the combined signal on the transmit channel.
Although the classical echo canceller works quite well for a white noise source, when the echoed signal is a highly correlated digitzed speech signal, convergence of the estimate with the echoed signal may be too slow to result in effective cancellation. S. Yamamoto et al., "An Adaptive Echo Canceller with Linear Predictor", Trans. IECE Japan, 1979, PP. 851-857, describes a scheme for overcoming this problem; wherein the coefficients derived from the received digitized speech signal for use in generating the adaptation coefficients are generated by the technique of prewhitening the received speech signal with linear prediction coefficents derived from the received speech signal in order to decorrelate the speech-representive components. This technique provides better convergence.
Echo cancellers must also be able to adjust for false adaptation which occurs when both transmit and receive speech signals occur simultaneously. This condition is known as "double talk." K. Ochiai et al., "Echo Canceller with Two Echo Path Models", IEEE Trans. COM-25, 1977, pp. 589-595 describes a system for overcoming false adaptation caused by double talk; wherein foreground and background estimates of the echo signal are generated separately and the parameter values used for generating the foreground estimate are refreshed by the parameter values used for generating the background estimate when a control logic circuit determines that the background estimate is providing a better approximation of the echo path transfer characteristics.